Many upper respiratory infections in cats are caused by Herpes virus which can be a mild chronic infection of the eyes and/or nose. If it is only occurring on one side of her face, make sure your veterinarian checks her well for any possible dental problems. A polyp can also form in the nasal sinus and cause chronic symptoms.
In some cases, L-Lysine can help with Herpes virus. It is not completely proven to work but I have seen many patients do better when given lysine regularly. There are some veterinary preparations of it that make it pretty easy to give to cats.
Also be sure to feed a high quality canned cat food and provide humidity in the house. Saline drops into the eyes and nose can help as well. Sometimes more specific medications such as antibiotics and antivirals are needed.
I hope this helps and your cat does better. It can be a frustrating condition to deal with.
Judy Karnia, DVM
Scottsdale Cat Clinic